Method of separating melamine from waste gas



March 7, 1957 ATSUO MURATA ETAL 3,308,123

METHOD OF SEPARATING MELAMINE FROM WASTE GAS Filed July 7, 1964ATTORNEYS United States Patent 3,308,123 METHOD OF SEPARATIN G MELAMINEFROM WASTE GAS Atsuo Murata, Tokyo, Ryo Kokubo, Mizuhiko Naga- 'ice isuniformly blended, cooled and solidified, the tempera ture within theseparator being maintained at over 60 C., thus yielding the solidifiedmelamine in the form of slurry.

kura, and Toshiro Ohba, Neigun, Toyama, and Mitsuo 5 i above Process isnot y economical as an Arata, Urawa, Saitama, Japan, assignors to Nis anpreclable percentage of hydrolysis products such as am- Kagaku KogyoKahushiki Kaislna, Tokyo, Japan Incline, ammelide and cyanuric acid areformed when the Filed .luly 7, i964, Ser. No. 380,876 molten melamine israpidly cooled with water; moreover Claims P y, application p July 18,1963, the purity and yield of melamine are low.

1 Claim 10 Ammonia decomposition of melamine takes place rapidly abovethe melting point of melamine but below the This invention relates to anindustrially useful process melting PoiIlt it hardly oooufsy Contrast,the hydrolysis for separating melamine from waste gas, particularly fromof melamine is y fast even beloW the melting Point, 50 Waste gas f r d ih h l ki f urea i that the loss in the wet separation process will begreater l i production I on account of hydrolysis than on account ofammonia It is extremely important to separate continuously anddecompositionon a large scale the molten substance mainly composed of Wehave found a Process for economically Separating melamine from NH and COgases in the production of melamine m a and 2 gases to give y P melamineby thermally cracking urea under pressure. melamine n good y We have fund that if m lt n However, melamine, which can remain stable in amolten melamine is r p y cooled with Water vapour or water statg above imelting point of 354 C under high presvapour-containing gas, hydrolysisdue to contact of water sure (particularly when the partial pressure ofammonia is Vapour With melamine Will be negligible, unlike the Case morethan 60 atm.) rapidly evolves ammonia if discharged When liquid Water iscontacted ith m lt n melamine. under normal pressure and producesundesirable impurities y utilising this effect We have inv nt d aneffective h as meklm d l semi-wet process of melamine separation.

We have earlier proposed, in U.S. patent application The presentinvention provides a process for the separa- No. 348,369, filed March 4,1964 and now abandoned, a ti n of melamine from waste gases, whichcomprises indry process of continuously separating melamine fromtroducing a mixture of the molten substance, composed NH and CO gases byintroducing a reaction mixture of mainly f m lamin a d ga us ammoniaandcarb the molten substance mainly composed of melamine and dioxideinto a separator, spraying the molten substance NH and CO gases into aseparator, Where the cool arri- With water, the amount of water sprayedbeing so conmonia gas is rapidly cooled by the circulated cool gastrolled that the average diameter Of the water drops iS (i.e. the gaswhich is cooled after separation from less than 200;]. and thetemperature in the separator is 90 melamine and then circulated);collecting the melamine to 200 C., simultaneously cooling andsolidifying the powder at the bottom of said separator andsimultaneously molten substance and discharging the solidified melaminecooling and solidifying the powder, said melamine powder from theseparator. being continuously discharged out of the separator and Tfurther explain the effectiveness of the present inthe NH and CO gasesbeing exhausted at the side of the vention, some experimental results onthe dry separation separator, process using cool ammonia gas, the wetprocess using This separation process is found to be the best methodWater, and the cooling Separation Process using Water as thedecomposition of melamine can be prevented and vapour-containing ammoniagas are described below in a high yield of high purity melamine can beobtained. order to compare the purity and yield of melamine ob- However,a drawback is that an enormous quantity of cool tained by differentprocesses. Urea at a rate of 12 kg./ hr. ammonia gas, or coolcirculating gas, is required for its and ammonia at a rate of 5 kg./hr.were passed under a industrial application on a large scale, because ituses pressure of 120 kg./cm. into an upright cylindrical reactgases withsmall heat capacity for the purpose of cooling ing vessel, of 120 mm.inner diameter X 90 mm. high, at the reaction mixture. its bottom; thenthe vessel was heated so'as to keep the The consumption of a largeamount of cool ammonia temperature at 420 C. From the top of the vessel,a gas or circulating cool gas requires not only a large reaction productcomprising molten melamine and NH; separator but also a large collectorfor melamine powder and CO gases was removed and introduced into aseparafrom the separator and a large separator and other accestor, wherethe molten melamine was cooled and solidified sories for separation ofNH and CO from the discharged by the cooling medium such as cool ammoniagas, water gas. Thus, the process has economical drawbacks. orWater-vapour-containing ammonia gas depending on A prior art wet processof separating melamine from the process. Melamine was obtained from thebottom of waste gas has been proposed. In this process a melaminetheseparator. separated aqueous solution is passed into a separator, Thecompositions and yields of melamine obtained are where the moltensubstance mainly composed of melamine given in the following table:

Composition Separator Cooling Medium temper- Yield ature, C. Mela-Hydrolytic Melam mine products 0001 ammonia gas 96.9 0.2 1.2 97.3 Do96.3 0.2 1.6 96.9 Water 70 98.7 6.4 2.1 90.6 Mixture of 70% ammo and 30%water vapour." 130 95. 8 0. 5 1. 5 96. 3 Do .7 95.9 0.5 1.5 96.3

The hydrolytic products are mainly ammeline and ammelide and include asmall amount of cyanuric acid. What we called the hydrolytic products inthe dry separation process using cool ammonia gas seems in reality notto occur within the separator but to have already been formed in thepressure reacting vessel.

As is seen from the results given above, the dry separation processusing cool ammonia gas can produce melamine in the highest purity andyield; the wet separation process using water is not of great practicaluse as it gives hydrolytic products to an extent of several percent. Thecool separation utilizing a water vapour-containing ammonia gas, whichinevitably gives some hydrolytic products, is inferior to the drycooling process using cool ammonia gas in both purity and yield ofmelamine obtained. However, the degree of inferiority in this lastprocess is less than 0.5% at the most, which value is considered to benegligible, and this is easily tolerated for industrial application ofthe process, without any great disadvantage.

Of course, the cooling separation process (i.e. using a watervapour-containing ammonia gas) will have the drawback that such gasrequires the same amount of cooling gas as does the cool ammonia gas.

However, we have made use of the advantage of the cooling separationprocess using water vapour or a water vapour-containing ammonia gas,namely that melamine suffers only slight hydrolysis, in the process ofthe invention.

The process can be carried out as follows: Water is sprayed into theseparator in such manner that it can blend uniformly with the reactionproduct comprising molten melamine and NH and CO gases, the sprayedamount of water being so controlled that the temperature in theseparator is kept higher than 90 C. and as the result the sprayed watermay be turned entirely into vapour; thus the reaction mixture is rapidlycooled by the latent heat of evaporation, thereby solidifying themelamine content and economically separating it from waste gas.

The water sprayed into the separator comes into contact with NH and COgases at least 350 C. and instantaneously evaporates, while the moltenmelamine is cooled by NH and CO gases which have been cooled to a lowtemperature. Thus, it is vital to keep the average diameter of thesprayed water drop at less than 200,1. maximum.

When the diameter of water drop is less than 200,11. evaporation willoccur very speedily and in consequence there will be little chance ofthe molten melamine coming into actual contact with water; this meansthat practically all the molten melamine is cooled by the NH and COgases, the greater part of which is constituted by water vapour, thuscausing little hydrolysis of the melamine.

If the average diameter of the sprayed water drop becomes larger than200 evaporation is retarded, giving much change for contact betweenmolten melamine and water drop to occur accordingly promoting thehydrolysis of melamine.

The amount of sprayed water should be so controlled that the temperaturewithin the separator is kept in the range of 90 to 200 C. Even if thetemperature is below 90 C., all of the sprayed water must be evaporatedso long as it is above the dew point. However, owing to the long timeneeded for evaporation, the chances of molten melamine (or evensolidified melamine) coming into contact with water drops are increasedat relatively high temperatures, and as the result melamine is liable tosuffer hydrolysis. Moreover, the operation of the separator is oftenhampered by condensation of water on its side walls or at its base.

At temperatures above 200 C., the rapid-cooling efiect is insufficientand in consequence the ammonia decomposition and hydrolysis of melamineare promoted. The

amount of water sprayed into the separator depends on the total of theamount of ammonia and the urea in the pressure reacting vessel. Thewater consumption may, however, be calculated more easily than thethermal balance. For instance, when the amount of ammonia is half thatof urea, the amount of water to be sprayed to keep the separatortemperature at 150 C. will be equal to about 30% of the amount of urea.

With an increased addition of ammonia, the sprayed amount of waterincreases and vice versa. The discharged NH and CO gases from theseparator with water vapour will amount to a fraction of the dischargedgases from the separator in the dry separation process using coolammonia gas or circulated cool gas; therefore the required separator andother accessories are very small, thus making the separation of melaminefrom waste gas very economical.

The following example illustrates the invention:

In the single figure of the accompanying drawing, 1 is an ammonia supplypump, 2 an ammonia heater, 3 a urea supply pump, 4 a pressure reactionvessel, 5 a spray tube for molten melamine, 6 a separator, 7 a waterfeed pipe, 8 a bag filter and 9 a gas discharge tube.

Molten urea is passed from the supply pump 3 at a pressure of kg./cm.and at a rate of 6-0 kg./hr., while ammonia is passed at a pressure of120 kg./cm. and at a rate of 30 kg./hr. from pump 1. These reactants areheated to 420 C. and then sent to the reacting vessel. The contents ofthe vessel are adequately heated so that the temperature in the vesselis 420 C. The vessel is so designed that it can retain the contents longenough for most of the urea to be converted to melamine. From the top ofthe vessel a reaction mixture of the molten substance consisting mainlyof melamine and NH and CO gases is introduced into the separator 6. Aspray tube sprays the mixture into the separator 6. The water is sprayedinto the separator at a rate of about 17 litres/hr. in such manner thatit can uniformly blend in eddy formation with the spray of the mixture,the average diameter of the water droplets being 50 The temperature inthe separator is held at C. and the cooled solidified melamine powder isdischarged at the bottom of the separator. The water vapour-containingNH and CO gases are exhausted at the side of the separator via dischargetube 9 so that small tiny particles of melamine are retained by the bagfilter 8.

The quantity of crude melamine retained by the separator and bag filteris 22.0 kg./hr., its purity being 96.0% by weight and its yield 96.8% byweight. The amount of melam contained in the crude melamine is 1.7% andthe hydrolytic products amount to less than 5% by weight.

What we claim is:

A process for the separation of melamine from waste gases, whichcomprises introducing a mixture of the molten substance, composed mainlyof melamine, and gaseous ammonia and carbon dioxide into a separator,spraying the molten substance with water, the amount of water sprayedbeing so controlled that the average diameter of the water drops is lessthan ZOO/.4. and the temperature in the separator is 90 to 200 C.,simultaneously cooling and solidifying the molten substance anddischarging the solidified melamine from the separator.

References Cited by the Examiner UNITED STATES PATENTS 2,918,467 12/1959Hibbitts 260249.7 3,132,143 5/1964 Fogagnolo 260-249] WALTER A. MODANCE,Primary Examiner.

J. M. FORD, Assistant Examiner.

